pansion of shrimp culture into many inland areas in order to reduce disease risk (Fast and Menasveta, 2000). In coastal areas, many shrimp ponds have been abandoned in Thailand and some other parts of the world due to diseases, poor management such as overstocking, and environmental degradation. The polyculture of shrimp-tilapia at relative low stocking density may provide an opportunity to develop a sustainable aquaculture system is the best use of abandoned shrimp ponds in coastal areas and low-salinity shrimp ponds in inland areas.
In a polyculture setting, shrimp and tilapia can utilize different niches. In extensive culture, tilapia can filter feed on phytoplankton and zooplankton in the upper water column, while shrimp spend most of the time in the pond bottom grazing on bacterial films on the bottom substrate and on the detritus settling from above. In intensive culture receiving pelleted feeds, tilapia may monopolize the feed, especially for floating feed. However, some feed particles always get to the bottom where the shrimp will get it. More importantly, the fecal matter from the tilapia contributes to the detrital rain that supports the shrimp. Akiyama and Anggawati (1999) reported that yields of shrimp increased when red tilapia (Oreochromis spp.) were stocked into existing shrimp ponds. The author suggested that stocking rate was 20Ð25 g fish per m2 and fish size at stocking was 50Ð100 g per fish. Akiyama and Anggawati (1999) reported that red tilapia assisted shrimp performance by improving and stabilizing the water quality, by foraging and cleaning the pond bottom and by having a probiotic type effect in the pond environment. Tilapia, as a filter feeder, can reduce excessive phytoplankton biomass in later stages of pond culture and recycle nutrients effectively (Stickney et al., 1979).
Nile tilapia (O. niloticus) is the most commonly cultured species among tilapias in many countries, including Thailand. Although Nile tilapia is the least saline-tolerant species among the commercially important tilapia species, Nile tilapia may be the tilapia species of choice in brackish water shrimp ponds, where the release of tilapia to estuaries is undesirable because Nile tilapia is unlikely to reproduce at salinities higher than
25 ppt (Teichert-Coddington et al., 1997).
Therefore the purpose of this study was to determine optimal conditions for stocking and rearing tilapia and shrimp in a polyculture system at low salinity.
Methods and Materials
Two experiments were conducted at the Asian Institute of Technology (AIT), Thailand for 65 and 75 days from 20 February to 26 April 2002 and from 27 November to 10 February 2003, respectively, to investigate growth performance
of shrimp and Nile tilapia, and water quality at different stocking densities of Nile tilapia in tilapia-shrimp polyculture. Nine
200 m2 earthen ponds were used for each experiment. There were three treatments in triplicate each for both experiments: shrimp alone at 30 per m2 (monoculture, control); shrimp at
30 per m2 and Nile tilapia at 0.25 per m2 (low tilapia density polyculture); shrimp at 30 per m2 and Nile tilapia at 0.50 per m2 (high tilapia density polyculture). Feed rations were varying in each pond and estimated by feeding trays (50 50
10 cm) daily in experiment one, while the same feed rations were used for all ponds in experiment two, which were determined by a typical feeding table (Lin, 1995) and the estimated average shrimp survival in experiment one.
Juvenile shrimps (PL15) were nursed in a 200 m2 earthen pond at AIT for 45 days prior to both experiments. The nursed shrimps (0.4Ð1.2 g) were stocked in all experimental ponds on 20 February 2002 for experiment one and 27 November 2002 for experiment two, while sex-reversed male Nile tilapia fingerings (5.5Ð8.0 g) were stocked seven days after stocking shrimp.
Prior to the start of the experiments, all ponds were drained completely, dried for two weeks, and filled with freshwater from a nearby canal to a depth of 80 cm. Then, the hypersaline water (150Ð250 ppt) was added to all ponds to adjust the salinity level to 5 ppt. Water level in all ponds was maintained at 1.0 m by adding fresh water biweekly to replace water loss due to seepage and evaporation. There was no water exchange throughout the entire experimental periods. The ponds were fertilized once before stocking shrimp by using urea at a rate of 28 kg N ha-1 wk-1 and triple superphosphate (TSP) at a rate of 7 kg P ha-1 wk-1. No other chemicals were used in the experiments.
One air blower (5 hp) was used to supply air for all experimental ponds. PVC pipes 2.54 cm in diameter were connected to the outlet of the air blower and extended to the pond dike of each pond. A polyethylene (PE) pipe 18 m long and 1.6 cm in diameter was connected to the PVC pipe and extended to the bottom of each pond. On the PE pipe, there were 10 holes of 1.5 mm in diameter, and the distance between two adjacent holes was 2 m. Nine 3 m long PE pipes were fixed alternately on the main PE pipe at the middle of two holes with four on one side and five on the other side. On each 3 m long PE pipe, there were two holes of the same size as those on the main PE pipe. The PE pipes were kept at 10 cm off the bottom using bamboo sticks. Aerators operated daily for 24 hours except during feeding.
Shrimps were fed commercial pelleted shrimp feed contain